For v1.14, as soon as a freighter accepts the contract for a given installation, even if it is in a different system to the pickup point, the supply and demand amounts for the start and destination populations are adjusted accordingly. This makes it much easier to see if the contracts are being met, even though the involved ships may still be underway, and eliminates the potential for oversupply.
I think this is already the case...at least, I have observed it to be the case when the freighter is in the same system as the supply site. (I haven't tested for inter-system effects.)
The problem I have observed is that when a freighter finishes loading at the supply site, the
Assigned amount on the
demand contract decreases by 1.
As a result, if additional supply is available (either currently, or becomes available before this freighter finishes this delivery), another available freighter will be assigned to this same demand unit.
I don't think the Assigned amount on the delivery contract should decrement until the freighter completes the delivery.